This invention is directed to a crystallizer, a process and an apparatus which are particularly suitable for producing high purity sodium chloride.
As discussed in detail in Sodium Chloride: The Production and Properties of Salt and Brine, Dale W. Kaufmann, 1971, pages 15-21, sodium chloride crystals are generally in the form of a simple cube without any modifying faces. However, the presence of certain foreign substances in the solution from which salt is crystallizing may cause modifications. Skeleton-type crystals (either octahedrons or combinations of octahedron and cube) may result from rapid crystal growth without the presence of a foreign substance. Further, hopper-shaped cubes can be produced by rapid crystal growth which is parallel to octahedron and dodecahedron faces. By filling out the cube edges and corners, hopper-shaped depressions are formed at the center of each of the cube faces. Such hopper-shaped cubes are particularly preferred where rapid dissolving is desired, such as in some food and seasoning applications and certain agricultural and chemical uses.
High purity is another frequently desired characteristic of sodium chloride. High purity may be desired in combination with rapid dissolution, as in the situations mentioned above, or high purity may be wanted for uses such as water softening.
Historically, high purity sodium chloride is manufactured in heated enclosed evaporators to vaporize solvent water. Production of brine from sodium chloride requires energy, typically obtained from fossil fuels. In order to reduce fossil fuel usage in sodium chloride production, various arrangements of brine heaters and evaporators have been designed and operated. Typical arrangements to enhance efficiency employ multi-effect evaporation and vapor recompression.
Sodium chloride produced in steam heated enclosed vessel evaporators is usually granular in size and cubic in crystal geometry. However, as mentioned above, sodium chloride crystal geometry is not necessarily cubic. For example, the Alberger process and the grainer process produce hopper-shaped crystals by open pan methods. Dentritic salt may be produced in enclosed vessels by introducing foreign substances (crystal habit modifiers) into the contained brine. Flat flakes may be produced by the compression of granular sodium chloride.
Solar vaporization of the aqueous solvent in a brine to produce "solar salt" is an ancient, low-cost alternative to the earlier presented methods of sodium chloride production. In its simplest form, solar salt is produced by exposing aqueous sodium chloride brines to sunlight, evaporating some or all of the aqueous solvent to cause concentration and thus crystallizing dissolved constituents (such as sodium chloride). The major disadvantage of solar salt is the reduced sodium chloride purity when compared to vacuum evaporated sodium chloride.
Another well-known process, sometimes, referred to as the "salting out" process, provides a relatively low energy, low cost method for the production of sodium chloride. In a salting out process, two or more aqueous solutions, each containing a single solute, or more typically, multiple solutes, are combined. When combined, the resulting aqueous mixture contains two or more solutes such that the solubility of one or more of the solutes is exceeded. For example, U.S. Pat. No. 3,832,143 discloses methods for making table-grade sodium chloride by mixing two brines having two distinct magnesium chloride concentrations, but each substantially saturated with respect to sodium chloride, to form a crystal crop of table-grade sodium chloride and a brine depleted in sodium chloride. The saturated brines may be prepared by solar evaporation of initial or starting brines such as ocean brines and Great Salt Lake brines. Specifically, Example II of U.S. Pat. No. 3,832,143 teaches mixing in a reactor crystallizer a first brine containing, among other things, 1.2 wt. percent Mg, 8.0 wt. percent Na and 14.8 wt. percent Cl and a second brine containing, among other things, 7.4 wt. percent Mg, 0.6 wt. percent Na and 20.2 wt. percent Cl. First a slurry, then substantially pure NaCl, are produced. U.S. Pat. No. 3,832,143 also discloses recycle of the depleted brine through the solar evaporation system. It should additionally be noted that U.S. Pat. No. 3,772,202 discloses use of a solar pond to concentrate a bitterns brine which contains NaCl but predominates in magnesium chloride. U.S. Pat. No. 3,852,044 discloses a solar evaporation system which produces sodium crystals, potassium minerals and an aqueous solution concentrated at least near magnesium chloride saturation.